Concentrated alcohol-based solutions are conventionally added to water in automotive cooling systems so as to provide anti-freeze protection. These water/alcohol heat transfer fluids are further inhibited from attack on the metal forming the automotive cooling systems by numerous corrosion-inhibiting additives.
The use of inorganic sodium silicates as corrosion-inhibiting agents is well known. However, sodium silicates tend to gel when used in corrosion-inhibiting effective amounts in alcohol-based coolant solutions. This "gelation" of the corrosion-inhibiting inorganic sodium silicates is problematic since the corrosion-inhibiting effectiveness of the silicate is detrimentally affected. The art has thus attempted to solve the gelation problem by various additives which serve to counteract the tendency of inorganic sodium silicates to gel in alcohol-based antifreeze solutions as evidenced, for example, by U.S. Pat. Nos. 4,149,985, 4,457,852 and 4,44460,478.
The present invention is directed to minimizing (if not eliminating entirely) the tendency of inorganic sodium silicates to gel in alcohol-based antifreeze solutions while simultaneously offering maximum corrosion-inhibiting effectiveness. Broadly, therefore, the present invention is directed to novel anti-corrosion coolant solutions for automotive cooling systems which include a synergistic corrosion-inhibiting effective amount of a sodium silicate having an unusually low ratio of silica to sodium oxide. More specifically, the present invention is directed to alcohol-based liquid solutions for automotive cooling systems which include an anti-corrosive effective amount of (i) a sodium silicate corrosion inhibitor having a ratio of silica (SiO.sub.2) to sodium oxide (Na.sub.2 O) of greater than 1.0 to about 2.5 (preferably between about 1.8 and 2.2).
The sodium silicate is typically employed in the alcohol-based liquid coolant system solutions of this invention in an amount sufficient to yield between about 0.01 to 0.2 wt. % silica (more preferably between 0.05 to about 0.06 wt. % silica) based on the total weight of the liquid solution.
The solutions according to the present invention may contain other additives conventionally employed in anti-freeze concentrates. For example, inorganic salts (e.g., sodium phosphate) may be employed in minor amounts up to about 1.5 wt. % based on the total solution weight.
While not wishing to be bound by particular theories, it is believed that by controlling the R value, it is also possible to reduce the corrosion of aluminum. By varying the SiO.sub.2 to Na.sub.2 O ratio (R), the corrosion rate is significantly minimized at an R of greater than about 1.0 to about 2.5, and most preferably about 1.8 to about 2.2.
Commercial antifreeze/coolants generally have polarization resistance (R.sub.p) values in the range of about 10.sup.5 to 10.sup.6 Ohms/cm.sup.2 . The degree of polymerization of silicate may be a function R. Aqueous silicate structure theory has been discussed in Iler, The Chemistry of Silica, Chapter 2, John Wiley & Sons, N.Y., 1979, hereby incorporated by reference.
At 1.0 R, the silicate of N=1 is essentially monomeric. The monomer provides very little corrosion protection. At 2.0 R, a silicate dimer may exist (N=2). At R values above 1.0 and below 2.0, a mixture of monomers and dimers may exist. This species forms a particularly stable film.
At an R value of above 2.3 to about 3.0, the N value is 15. It is believed that a geodesic sphere containing SiO.sub.2 groups forms. This geodesic sphere is a weak inhibitor.
Further aspects and advantages of this invention will become clearer after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follows.